1. Field of the Invention
The present invention relates to a method and system for estimating the location of a wireless device, more particularly to a method of inexpensively improving the accuracy of the estimation.
2. Description of the Related Art
Estimating the location of a target device from the known locations of a plurality of anchor devices on the basis of the strengths of wireless signals transmitted and received by these devices is a known art. The estimation process makes use of a set of parameters such as propagation coefficients and correction coefficients describing signal propagation characteristics in the location estimation area. These parameters can be used to calculate distances from the target device to the anchor devices, or to model expected signal strengths at different locations and optimize the model by finding the location that best matches the actual received signal strengths.
For example, a system disclosed in U.S. Pat. No. 6,473,038 to Patwari et al. (and in corresponding Japanese Patent Application Publication No. 2005-507070) determines the unknown locations of a plurality of devices by measuring signals sent among these devices and signals sent between these devices and devices having known locations. The measurements are input to a graph function with a plurality of first sub-expressions and a plurality of signal measurement estimation sub-expressions having extrema when predicted signal measurements are equal to the actual signal measurements, and the graph function is optimized.
A problem with such systems is that the signal propagation parameters may vary greatly within the location estimation area. Outdoor and indoor signal propagation environments, for example, have different parameters, and the parameters of indoor environments also differ depending on room size and shape, ceiling height, wall, ceiling, and floor materials, and the presence of furniture and other paraphernalia. Even within a single room, the propagation parameters may vary from place to place, or from time to time as doors are opened and shut and people and objects enter, leave, or move around. All of these factors reduce the accuracy of conventional location estimation systems that model the location estimation area as a uniform plane with constant propagation parameters.
This problem can be partly overcome by dividing the plane into sub-areas, as proposed in Japanese Patent Application Publication No. 2006-329688, for example, and using different parameters for each sub-area. This scheme becomes costly, however, because it requires anchor devices to be installed in each sub-area. Moreover, it still relies on the unrealistic assumption that each sub-area is a uniform space with constant propagation parameters. A further problem is the additional computation needed to determine the sub-area in which the target device is located.